Photonic circuits in which beams of light redirect the flow of other beams of light, are a long-standing goal for developing highly integrated optical communication components. Ideally, circuits based on optical interconnects would be constructed using sub-micron-size devices in which photons are manipulated in a manner similar to that how electrons are manipulated in a semiconductor electrical circuit. Furthermore, it is highly desirable to use silicon, the dominant material in the microelectronic industry, as the platform for these photonic chips. Photonic structures that bend, split, couple and filter light have recently been demonstrated in silicon, but the flow of light in these structures is predetermined by the structure design and cannot be modified.
All-optical switches and modulators have been demonstrated with III-V compound materials based on photo-excited free-carrier concentrations resulting from one or two photon absorption. However, in silicon, all-optical switching has only been demonstrated in large, out-of-plane structures using very high powers. High powers, large size, and out-of-plane geometries are inappropriate for effective on-chip integration. The difficulty in modulating light using silicon structures arises from the weak dependence of the refractive index and absorption coefficient on the free-carrier concentration. For example, a 300 μm long Mach-Zehnder modulator based on rib waveguides with mode-field diameter (MFD) of about 5 μm, a minimum optical pump pulse energy of 2 mJ is needed to modify the refractive index by Δn=−10−3 in order to achieve 100% modulation. The absorption due to free-carriers under such high powers is also small (16 dB/cm for a 450 nm wide and 250 nm high rectangular cross section waveguide) which demands a straight waveguide as long as 600 μm in order to achieve modulation depth of 90%.